Rechargeable lithium batteries are becoming increasingly more common in the market place. This is a result of greater demand for higher energy density power sources for electronics applications and of recent improvements in the technology. A new type of battery based on lithium ion (also referred to as "rocking chair") technology was recently made commercially available by Sony Energy Tec (T. Nagaura et al., Progress in Batteries and Solar Cells, 9, 209, (1990)). This system uses two, suitably chosen, intercalation compounds as electrodes each of which acts as a host for lithium. It is thus desirable to select materials that are capable of reversibly intercalating large amounts of lithium per unit host material. Since the battery voltage is determined by the potential difference between the two electrodes, it is desirable to select two materials that differ significantly in potential.
Typically, the cathode in such systems is a lithium transition metal oxide such as LiCoO.sub.2 (Goodenough et al., U.S. Pat. No. 4,302,518) or LiMn.sub.2 O.sub.4 (Thackeray et al., U.S. Pat. No. 4,507,371). These materials reversibly intercalate lithium in a potential range around 3 to 4 volts with respect to lithium metal. The anode material employed is typically a carbonaceous material with a graphite or disordered graphite structure. These materials reversibly intercalate lithium in a potential range mainly within a few hundred millivolts above that of lithium metal.
The use of graphite as an anode material has been disclosed previously in inventions by Sanyo (Japanese published examined patent application No. 87023433) and by Basu (U.S. Pat. No. 4,423,125) amongst others. Highly crystalline graphite offers large theoretical reversible lithium capacity. Herein, we define reversible capacity as the amount of lithium, .increment.x, in Li.sub.x+.increment. C.sub.6 which can be reversibly intercalated over many cycles. It is commonly believed that pure graphite can reversibly intercalate .increment.x=1 worth of lithium. This corresponds to 372 mAh/g capacity for the graphite.
Although graphite has excellent capacity properties, difficulties are encountered in its practical application in lithium ion cells. Lithium in all carbonaceous hosts is lost to some extent. This loss of lithium is irreversible, hence it is important that it be minimized. The mechanism for all the losses observed are not completely understood. To some extent, lithium in a carbonaceous anode reacts at the anode surface with the electrolyte used in the battery. When graphite is used as an anode, the irreversible loss, which occurs when many common electrolytes are used, is very large and unacceptable. This is believed to occur in some cases as a result of electrolyte solvent co-intercalating with the lithium into the graphite. The graphite presumably exfoliates when the large solvent molecules co-intercalate, which creates more anode surface area, which in turn increases the amount of lithium which can react with the electrolyte. D. P. Wilkinson et al. (U.S. Pat. No. 5,130,211) disclose the use of a sequestering agent that reduces the irreversible loss of lithium in a graphite anode when incorporated into a propylene carbonate solvent based electrolyte. Shu et al. (J. Electrochem. Soc. 140(4), 1993)) further show the unacceptable behaviour of a pure graphite anode for use in a lithium ion battery using propylene carbonate/ethylene carbonate (50/50 blend by volume) solvent based electrolyte. Addition of a sequestering agent again adequately reduces the irreversible loss of lithium. Matsushita (6th International Lithium Battery Conference, Muenster, Germany, May 13, 1992) reveals an ethylene carbonate based electrolyte that also significantly reduces these irreversible losses of lithium in batteries employing graphite anodes. Thus, there are electrolyte compositions that can be employed to allow practical application of graphite as an anode. Often however these compositions are not desirable for other reasons, including incompatibility with the cathode material, cost, safety, etc. Therefore, it is desirable to obtain at least the specific capacity of graphite in special carbons or in other materials that may not additionally restrict the choice of solvents employed in the electrolyte.
Another difficulty encountered in the practical design of a lithium ion battery results from the small potential difference between the lithiated carbonaceous material and that of lithium metal. During charging of a lithium ion battery, lithium is de-intercalated from the cathode and preferentially intercalates into the anode. If the overvoltages which occur during charge are too large (as a result of charging quickly), electroplating of the lithium may occur instead. Much of any plated lithium is effectively lost as the cycling efficiency of lithium metal is poor, causing a relatively rapid loss in cell capacity. Also, the presence of plated lithium presents an increased safety hazard. Thus, high-rate battery designs are required that result in low anode overvoltages. Of the possible carbonaceous materials for use as anodes, the potential difference between that of lithiated graphite and lithium metal is amongst the lowest.
Other carbonaceous materials can be suitably employed as anodes instead. Mitsubishi Petrochemical (U.S. Pat. Nos. 4,702,977 or 4,725,422) discloses other carbon materials useful as lithium ion battery anodes. Similarly, Moli Energy (U.S. Pat. No. 5,028,500) discloses further carbonaceous anode materials. These materials offer certain advantages over pure graphite that include a greater potential difference with respect to that of lithium metal and the option to use other suitable electrolyte compositions. However, the reversible capacity of these materials is not as great as that of graphite. (Those skilled in the art are aware that a greater potential difference with respect to lithium metal results in a lithium ion battery with lower operating voltage. However, a relatively large increase in the lithiated carbon potential with respect to lithium metal can be achieved with only a relatively small reduction in battery operating voltage. Thus, the energy density of the lithium ion battery is sacrificed only to a small extent in return for a large possible gain in the battery rate capability).
Composite anode materials have recently been investigated wherein the materials mainly consist of carbon but also contain boron to some extent. Sony (Japanese patent application laid open No. 03-245458) discloses the use of a carbonaceous material containing 0.1-2.0% by weight boron as an anode in a rechargeable battery. The addition of varied amounts of boric acid in the synthesis of the material resulted in anodes with differing lithium capacity. A definite but small increase in capacity similar to that of graphite was obtained for a preferred composite anode wherein approximately 1% boron by weight remained in the composite. In this reference, this preferred composite anode attained deliverable capacities on discharge of 380 mAh/g compared to the 310 or 350 mAh/g of the two respective comparative examples shown where no boric acid was used in the preparation. This therefore corresponds to a 23% and a 9% capacity increase with respect to each comparative material shown. According to this reference, an increase in the amount of residual boron (up to 2.5% by weight) results in no gain in capacity. The voltage curves in the figures of this application show no apparent difference in the battery voltage and hence presumably no difference in the potentials between that of the invention example and the comparative example materials.
Several references also appear in the literature that mention composite boron carbon anode materials containing elements from other groups in the periodic table. Central Glass Co. (U.S. Pat. No. 5,139,901) discloses the use of a different anode composite which, in addition to boron, contains nitrogen and hydrogen. The preferred material provides only 97.3 mAh/g of reversible capacity making it relatively impractical for commercial use. Those skilled in the art recognize, from the available prior art, that the potential of carbonaceous materials with respect to lithium metal varies significantly with type of carbon. There is no indication in this patent application of Central Glass Co. that incorporation of boron or any of these other elements in the composite results in a material with a potential that differs from that of a similar carbon prepared without nitrogen or hydrogen. Morita et al. (J. Electrochem. Soc. 139(5),1227,(1992)) discuss the use of BC.sub.2 N as an electrode material. The electrode examples exhibited large polarization even at low rate making these electrodes impractical for use in commercial battery products. A substituted structure was postulated and mention is made that the operating potential of Li.sub.x BC.sub.2 N is somewhat higher than that of a carbon like petroleum coke. Again, there is no indication that a potential shift with respect to that of a similar carbon prepared without B and N was achieved. Also, in this reference, the higher operating potential of this composite is considered to be a disadvantage. In both these references, the element nitrogen which acts as an electron donor has been included in the composite along with boron. Sony (Eur. Pat. Appl. EP486950) mentions use of a composite carbon anode material that could include phosphorus or boron. However, this was merely a suggestion as a possible anode material and again, phosphorus acts as an electron donor.
Compounds of the form B.sub.z C.sub.1-z where boron has been substituted for carbon in the structure have been reported in the literature. As early as 1967, Lowell (Journal of the American Ceramic Society 50, 142 (1967)) Showed that carbons could be doped substitutionally with 2% boron in a high temperature synthesis (2400.degree. C.) involving B.sub.4 C and carbon. At these temperatures, no further boron beyond 2.3% atomic can be substituted for carbon. Later work by Kouvetakis et al. (J. Chem. Soc. Chem. Commun. p. 1758 (1986)), Kouvetakis et al. (Synthetic Metals 34, 1 (1989)), and Kaner et al. (Materials Research Bulletin 22, 399 (1987)) described how substantially higher doping levels could be achieved by a low temperature synthesis method. Kouvetakis et al. prepared material which they claim had a stoichiometry of BC.sub.3. Further work by the same group showed that sodium could intercalate into these materials. They suggested that boron substituted carbons could be useful as electrodes in lithium batteries, but did not demonstrate any advantages over the prior art. Furthermore, there is no evidence that these boron substituted carbons were ever tested for lithium intercalation, or that any electrochemical cells were constructed.
The inventors (B. M. Way et al. Phys. Rev. B. 46, 1697 (1992)) also confirmed that boron substituted carbonaceous materials can be made with z in B.sub.z C.sub.1-z as large as 0.18. No electrochemical data was reported. Further material presented by the inventors (B. Way and J. R. Dahn, extended abstract #30, at the 1992 Fall Meeting of the Electrochemical Society, Toronto, Ontario, Oct. 12-16, 1992) show data for Li.vertline.B.sub.0.17 C.sub.0.83 cells which show a useful significant shift in potential of the anode material with respect to lithium when the carbon is doped substitutionally with boron. No capacity improvement was shown, however, over that published in the prior art.